And intelligence staff are working round the clock to track down the group, which is thought to contain up to 40 extremists, mainly from Pakistan.

They say a strike could come “at any time” and could even happen before the end of the Easter break.

Bomb

“Soft targets” would include football grounds, shopping malls or tourist attractions to ensure maximum casualties.

But MI5 officers believe the latest wave of terrorists is plotting an atrocity in London, not the North-West

The existence of the second cell was revealed to Cobra, the emergency committee of intelligence chiefs and ministers after last week’s arrests in Clitheroe, Lancs, Manchester and Liverpool.

Most of the suspects are in Britain from Pakistan on student visas and at least one is said to have links to a house owned by one of Osama bin Laden’s henchman.

The raids were launched when police received details of a plot to bomb Manchester’s Piccadilly train station over Easter.

The operation had to be brought forward when counter-terror chief Bob Quick, 49, accidentally revealed secret details of the suspects.

Meanwhile, the Real IRA has re-opened the chilling possibility of a second front in the war against terror.

It issued an Easter message yesterday warning it would restart attacks on mainland Britain “when it is opportune”.

The group – which murdered British soldiers Mark Quinsey, 23, and Patrick Azimkar, 21, outside the Massereene Army base in Northern Ireland last month – said it aimed to wreck the Ulster ceasefire by hitting targets in London.

The Real IRA last launched attacks in the British capital in 2000, when it struck MI6 headquarters with a rocket and detonated a car bomb outside a BBC office.

The group also threatened to kill Sinn Fein deputy leader Martin McGuinness.

It criticised McGuinness, 58 – a former IRA chief – for backing the peace process and said: “No traitor will escape justice regardless of time, rank or past actions.”